In my office I have two machines connected two networks - one has IP say 21..... and another has IP say 25... Now the thing is that both of the machines can access Internet. But there is a problem pinging among each other. I have been told that has been done on purpose. I am not sure what is going on here. If there is no private VLAN issues there and if each machine has an IP address then why would not they be able to ping each other. Well this is not an issue here, I need to understand the mechanism here.

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sorry to interrupt, just wanna know the basics of networking.

Hello, Read your post & I dont understand your pinging problem either. You say ther are 2 p.c.'s connected to 2 networks? I'm a part of a VPN, the server is a 100 miles away & we connect through a couple network switches, a sisco router/ firewall as the gateway, then through TW hardware on 2 T1 lines. All our p.c.'s/ hardware have their own IP address in the 10.102.x.x range & I can ping every piece of hardware here. Do both those machines connect to the same router/ gateway? Is the cabling straight through? Interesting. Anyone have a handle on this kind of network?

I'm going to assume you have two PC's, one with an IP address similar to and the other

In this case both PC's are on the same network.

I'm thinking that ICMP (Internet Control Message Protocol) traffic is being blocked either from the installed firewalls on your PC's. ICMP is how ping and trace-route (tracert in windows) communicate. If it's not your PC's firewall blocking the ICMP it's your LAN switch blocking them.

If you are truly on two different networks where your IP's are in different subnets, for example PC1 is in and the PC2 is and your subnet mask is then it could be the router blocking the ICMP.

So the question comes down to why block ICMP? Because DoS (Denial of Service)attacks originally came from a multitude of machines pinging a target in order to overwelm and over burden the target and its network so no one else could use it.

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